996 Monster 750 low compression check

[negativeION]

I've been working on a 1996 Monster 750 with only about 13k miles. I'm not totally sure that mileage is real but I checked the compression after warming up the engine. Both cylinders showed 120 psi or 8 bar. I even added a little oil to the cylinders and the reading did not change. Now 8 bar sounds low to me and it usually means an issue. But the service manual says 9 to 11 bar is standard and 8 bar is the minimum so technically it is still within the factory spec.

The engine only has a 9:1 compression ratio to start with. It is good that both cylinders match, and the oil test did not change the results. That strongly suggests the piston rings are fine. Then I checked the valve clearances. The horizontal cylinder's clearances were fine, but the vertical cylinder was a little loose, with one out of limits.

Since the compression numbers match for both sides, I don't think the loose valve is causing a problem. I am only going to replace that one bad shim for now. What do you all think about this motor, is it really good to run or should I be worried? What other easy checks should I do before putting it back together?

 

[Carter.M]

Those numbers don’t sound too alarming for a 9:1 engine, with both cylinders matching and no change after the oil test. If the clearances are sorted and it runs smoothly, I’d say it’s likely fine

 

[doremiiiiii]

What you've done so far is a strong evidence. The fact that both cylinders match this number is actually more important than the number itself.
While a slightly loose valve clearance on the vertical cylinder isn't ideal and should be adjusted, it's not going to raise the compression; if anything, a tight valve is more likely to cause low compression by holding a valve slightly open. Before sealing everything up, I would suggest a simple leak down test on both cylinders